Quantitative double echo steady state T2 mapping of upper extremity peripheral nerves and muscles

Abstract

Introduction: T2 mapping can characterize peripheral neuropathy and muscle denervation due to axonal damage. Three-dimensional double echo steady-state (DESS) can simultaneously provide 3D qualitative information and T2 maps with equivalent spatial resolution. However, insufficient signal-to-noise ratio may bias DESS-T2 values. Deep learning reconstruction (DLR) techniques can reduce noise, and hence may improve quantitation of high-resolution DESS-T2. This study aims to (i) evaluate the effect of DLR methods on DESS-T2 values, and (ii) to evaluate the feasibility of using DESS-T2 maps to differentiate abnormal from normal nerves and muscles in the upper extremities, with abnormality as determined by electromyography.

Methods and results: Analysis of images from 25 subjects found that DLR decreased DESS-T2 values in abnormal muscles (DLR = 37.71 ± 9.11 msec, standard reconstruction = 38.56 ± 9.44 msec, p = 0.005) and normal muscles (DLR: 27.18 ± 6.34 msec, standard reconstruction: 27.58 ± 6.34 msec, p < 0.001) consistent with a noise reduction bias. Mean DESS-T2, both with and without DLR, was higher in abnormal nerves (abnormal = 75.99 ± 38.21 msec, normal = 35.10 ± 9.78 msec, p < 0.001) and muscles (abnormal = 37.71 ± 9.11 msec, normal = 27.18 ± 6.34 msec, p < 0.001). A higher DESS-T2 in muscle was associated with electromyography motor unit recruitment (p < 0.001).

Discussion: These results suggest that quantitative DESS-T2 is improved by DLR and can differentiate the nerves and muscles involved in peripheral neuropathies from those uninvolved.

Keywords: T2 mapping; deep learning reconstruction; magnetic resonance neurography; peripheral neuropathy; quantitative MRI.

Figure 1

Flow diagram of subject inclusion, reasons for exclusion, and subjects analyzed.

Figure 2

DESS pulse sequence diagram and equation used for T2 calculation (A). The first echo, S+, utilizes free induction decay signal and has mixed T1 and T2 contrast, while the second echo, S-, is more heavily T2-weighted, due to contributions from previous excitations. Representative axial images from a 39-year-old man with Parsonage-Turner Syndrome of S+ (B) and S- (C) echoes acquired within the proximal forearm reveal diffuse hyperintensity, compatible with active denervation, of the pronator teres muscle (^*), best seen on the S- image, and also of the median nerve (MN, arrow). Corresponding DESS-T2 map (D) demonstrates increased T2 in the pronator teres and median nerve.

Figure 3

Comparison of DESS S⁻ images with standard reconstruction (A) and with DLR (B) from a 44-year-old man with ulnar neuropathy. Insets show reduced noise and superior conspicuity with DLR of fascicular bundles in the normal median nerve (MN) (C, E) and abnormal ulnar nerve (UN) (D, F). The effect of DLR is also seen in T2 map insets (G–J) corresponding to (C–F).

Figure 4

Oblique coronal multiplanar reformat (MPR) of 3D DESS acquired at the elbow joint (A) shows enlargement and hyperintensity (bracket) and a focal constriction (arrows) of the median nerve in a 48-year-old man, compatible with Parsonage-Turner syndrome. Axial DESS S- image (B) through the distal forearm shows muscle ROI selection of the pronator quadratus (PQ) (∧) and flexor carpi ulnaris (FCU) (^*). T2 map (C) through the distal forearm shows increased T2 in the denervated PQ compared to the normal FCU.

Figure 5

Oblique sagittal (A) and coronal (B) multiplanar reformat (MPR) of the 3D DESS S- echo, acquired at the elbow joint in a 51-year-old man with cubital tunnel syndrome, show the median (MN) and ulnar nerves (UN). A 3D rendering (C) generated using 3D DESS for nerves and a zero time-to-echo sequence for bones allows for visualization of the MN, UN, and radial nerve (RN).

Figure 6

Associations between mean T2 with DLR (A–D) or with standard reconstruction (E–H) and EMG metrics of denervation potentials [fibrillation potentials (FPs)/positive sharp waves (PSWs)] and motor unit recruitment (MUR). A significant, positive association was shown between abnormal nerves and MUR (A, E) and abnormal muscles and MUR (C, G). Associations between abnormal nerves and FPs/PSWs (B, F) and between abnormal muscles and FPs/PSWs (D, H) were not significant.